The present invention relates to a method of improving the mould quality of a mould-forming machine, such as a string-moulding apparatus, a flaskless mould-forming machine or the like, in which the mould-forming process comprises the steps of filling a moulding chamber with compressible mould material, e.g. clay-bonded green sand, pressing the mould material between two squeeze plates, thus forming the mould or mould parts (cope and drag), the moulding cavities being provided in the produced mould or mould parts by means of pattern plates. After pressing the mould material, the moulding chamber is opened by moving at least one of the squeeze plates and the produced mould or mould parts are removed from the moulding chamber. Thereafter, the moulding chamber is reclosed and ready for a new cycle.
A method of this general kind is known from U.S. Pat. No. 5,647,424. According to this method, the squeeze plates carry out a number of sequential movements in order to produce a mould. The moulding process comprises the steps of: Filling the moulding chamber with compressible mould material, e.g. clay-bonded green sand, pressing the mould material between two squeeze plates, thus forming the mould, retracting one of the squeeze plates for opening the moulding chamber and removing the produced mould from the moulding chamber and moving the squeeze plates back to their respective starting positions, whereafter a new cycle begins.
Document EP-0,468,355 discloses a similar method, but used in a moulding machine with a cope-flask, a pattern plate and a drag flask, for producing flaskless moulds, i.e. drag and cope.
In the above methods, the filling of the moulding chamber is started after closing the moulding chamber with the squeeze plates, the squeeze plates being stationary during the filling step, and the squeezing is started after the filling has been completed. The filling of the moulding chamber is performed by exposing the sand hopper to pressure or by evacuating the moulding chamber, thereby providing a pressure difference between the sand hopper and the moulding chamber, blowing the mould material into the moulding chamber. When starting the filling of the moulding chamber, the velocity of the mould material increases during the initial filling and accordingly, the filling of the part of the moulding chamber opposite of the filling opening will be less effective than the filling of the following parts of the moulding chamber. Thus, a difference in quality of the mould is experienced between the parts filled first and the parts filled later during the filling procedure. This may result in differences in the degrees of hardness of the mould surfaces produced in the different parts of the moulding chamber and differences in the effectiveness of filling recesses in pattern plates in the moulding chamber. These differences in hardness of the mould from top to bottom result in reduced dimensional precision and surface properties.
It is the object of the present invention to provide a method of improving the mould quality of a mould-forming machine of the kind referred to above, which alleviates the above-mentioned problems, thus resulting in a higher quality of the produced moulds. This object is achieved with a method of said kind, by the movement of at least one squeeze plate in a direction reducing the volume of moulding chamber during at least part of the filling step which results in an improved filling of recesses in the pattern plates, especially in the parts of the moulding chamber opposite the infeed opening(s).
According to a preferred embodiment of the invention, the movement of least one squeeze plate in said direction is performed during the initial part of the filling step. This will result in a pre-compression of only the first filled part of the moulding chamber, which is filled with mould material at a lower velocity than the rest of the moulding chamber.
In a further preferred embodiment, the movement of at least one squeeze plate is performed in continuation of the closing movement of said squeeze plate, whereby it will not be necessary to bring said squeeze plate to a halt before starting the filling operation.
In accordance with a further aspect of the invention, the movement of the squeeze plate during the filling step is performed without stopping but with varied, controlled speed during the filling step and the movement is continued into the pressing step.
Further preferred embodiments are revealed hereinafter.